A related art mobile phone system is predicated on a one-to-one correspondence relationship between base station and terminal, and any service for transmitting data simultaneously to two or more terminals by base stations is not expected. Conventionally, there has been provided a method of notifying information to terminals placed in a cell all at once using a common channel which is called broadcast channel. This method is for notifying control information to terminals, but is not for providing high-speed communications for users.
In recent years, multimedia services are expected as mobile communications services. Especially, there has been a growing interest in a technology for delivering multimedia information, such as a sportscast, a weather report, a radio broadcast, simultaneously to two or more users. Conventionally, there has been a necessity to provide required radio resources securely for every terminal for provision of information via one-to-one communications. A broadcast multimedia service is aimed at delivering one transmission information simultaneously to two or more users in order to save radio resources so that two or more users can receive data provided by this service and transmitted from a base station simultaneously using a common channel. In particular, this technology is called MBMS (Multimedia Broadcast Multicast Service) in 3GPP (3rd Generation Partnership Project).
The challenge to such a related art multimedia service is to guarantee the reception quality of a terminal which is staying in a cell's edge where the power of the received signal is weak. A method of securing the reception quality of a terminal by performing transmission power control is disclosed, as a solution to the challenge, by JP, 2003-188818,A. According to this method, the transmission power of a base station is controlled so that the reception quality of even a terminal received minimum reception power that is staying in a cell's edge can be guaranteed. A problem with the related art method is however that since the transmission power is increased for a channel which does not have any function of performing transmission power control originally, the operation of controlling the transmission power is complicated, and since the transmission power is controlled according to the minimum reception power of a terminal which is staying in a cell's edge, the transmission power becomes large easily.
In addition, there is a method of selective combining which is different from soft combining. According to this selective combining method, a terminal receives signals simultaneously from two or more base stations and selects one of the signals having the best quality. The method is excellent because it makes it possible to store data even if there is a delay which exceeds the window size of the terminal. However, since the terminal needs to receive a number of channels for selective combining which corresponds to the number of signals subjected to the selective combining according to this selective combining method, the terminal needs many receiving circuits and therefore the hardware scale of the terminal becomes large. Therefore, when a terminal having a small number of receiving circuits receives an incoming call via a new dedicated channel during selective combining, the terminal has to give up the selective combining and to continue receiving MBMS data using only a channel from one base station. Then, since the reception quality of the terminal degrades rapidly, the base station cannot but raise its transmission power. As a result, a big problem is that since power assigned to other terminals, such as dedicated channels to other terminals, decreases, the capacity of the base station is also reduced.